Electrophotographic materials

ABSTRACT

AN ELECTROPHOTOGRAPHIC MATERIAL HAVING AN ELECTROPHOTOGRAPHIC LAYER FORMED OF A FINELY DIVIDED INORGANIC PHOTOCONDUCTOR, ONE OR MORE SPECTRAL SENSITIZING DYES AND AN INSULATING FILM FORMING MATERIAL COMPRISING AN ALKYD RESIN CURED WITH A POLYISOCYANATE COMPOUND. THE ABOVE IS COATED ON A SUPPORT, AND THE ALKYD RESIN CONTAINS 10 TO 50% BY WEIGHT OF AN AROMATIC DIBASIC CARBOXYLIC ACID AND HAS A HYDROXYL VALUE WITHIN THE RANGE 10 TO 100. THE POLYISOCYANATE COMPOUND IS CONTAINED IN AN AMOUNT FROM ABOUT 1.2 TO 8 TIMES THE EQUIVALENT AMOUNT CORRESPONDING TO THE HYDROXYL VALUE OF THE ALKYD RESIN.

Dem 1972 SATORU ONJO ErAL 3,105,032

ELECTROPHOTOGRAPHIC MATERIALS Filed Aug. 20, 1969 LLI United States Patent 3,705,032 ELECTROPHOTOGRAPHIC MATERIALS Satoru Honjo, Masaaki Takimoto, Seiji Matsumoto, and Kuniko Kosuge, Asaka-shi, Japan, assignors to Fuji ghoto Film Co., Ltd., Minami-ashigara-shi, Kanagawa,

apan

Filed Aug. 20, 1969, Ser. No. 851,562 Claims priority, application Japan, Aug. 20, 1968, 43/59,400 Int. Cl. G03g 5/08 US. CI. 96-13 11 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention The present invention relates to an electrophotographic material sensitized with spectral sensitizers and, in particular, relates to an electrophotographic material having a novel binder composition which is suitable for liquid development.

Description of the prior art In electrophotography a process is known which comprises exposing an image onto a photoconductive insulating layer provided on a support after uniformly imparting an electrostatic charge thereon, thereby discharging the charge in the light-irradiated region to form an electrostatic latent image. Subsequently, the image is developed with a developer having dispersed therein an electrostatically charged finely-divided toner in a highly insulating liquid carrier (which does not destroy the latent image) to convert the latent image to a visible image. In a liquid development process as described above, the line powder having an electrostatic charge in the liquid carrier electrophoretically deposits on the latent image, thus making the latent image visible.

As an electrophotographic material, one prepared by establishing a photoconductive insulating layer having dispersed therein a finely-divided inorganic photoconductive material such as zinc oxide, cadmium sulfide, titanium dioixde, zinc sulfide, etc. in an insulating filmforming material on a flexible support, such as paper, is known.

Heretofore, binder compositions used in an electrophotographic material containing finely-divided inorganic photoconductors suitable for liquid developing processes include cured alkyd resins, polymethylmethacrylate, vinyl chloride/vinyl acetate copolymers, etc. All are insoluble in the liquid carrier of a liquid developer.

Although all the above-described binders exhibit good characteristics, in particular, the cured alkyd resins have excellent advantages such as being capable of forming a flexible sensitive layer showing little tendency to curl; of dispersing an organic pigment; of being stable in organic solvents; and of permitting the use of inexpensive organic solvents, such as toluol or xylol, in coating operations.

The inventors have found that the decay characteristics of an electrostatic charge on an insulating layer in a liquid developer which has heretofore been less noted is one of the most important factors affecting image quality. In particular, in the case of reproducing a continuous tone image, it has been proven that a slow decay of the surface charge in a liquid developer is essential for high quality image reproduction.

The cured alkyd resins have been found satisfactory as a binder with respect to the decay characteristic in the developer. Other resins which have hitherto been used as a binder form a hard coating and hence incorporation of plasticizers in a photoconductive coating containing such resins is ordinarily necessary to provide a photoconductive paper with an acceptable mechanical behavior. Plastici-zer incorporation, however, is sometimes accompanied by deterioration of the charge retention property in the liquid developer as well as a remarkable lowering of light sensitivity, resulting in an inferior material to those comprising alkyd resins.

Heretofore, the curing of alkyd resins has been carried out by using at least one of the organic peroxides such as methyl ethyl ketone-peroxide, benzoyl peroxide, etc. and heavy metal compounds such as naphthenates of cobalt, manganese, iron, zinc, lead, etc.

However, the alkyd resin binder composition has been found to be accompanied with a serious problem in a coating spectrally sensitized with organic sensitizing dyes, in particular, with those sensitive to oxidation, while there arises no problem in a coating composition not containing spectral sensitizers. It is clear that the sensitizers are gradually decomposed in the presence of a catalyst such as cobalt naphthenate. The decomposition seems to proceed considerably at the curing step of the resin, and in particular, sensitizers such as cyanine, merocyanine and xanthine dyes have been found weak to oxidation, and coatings containing such dyes faded rapidly during storage.

In order to avoid this difiiculty, we tried to decrease the amount of catalytic heavy metal compounds such as naphthenates of cobalt, manganese, lead, etc., to a very small amount or even to zero, while instead making the curing conditions more intense by raising the curing temperature or extending the curing time. This is because we found that even the presence of a very small amount of heavy metal compound is quite harmful and that very severe conditions of heat treatment are required to cure in the absence of such compounds. It has been shown decisively that a coating containing an alkyd resin binder insufliciently cured never exhibits a satisfactory charge retaining property in many nonpolar organic solvents suitable for the insulating carrier liquid of liquid developers.

Another serious problem lies in the fact that the alkyd resin-containing photoconductive coating cured in the presence of a heavy metal catalyst such as cobalt naphthenate exhibits a photographic tone reproduction characteristic curve with an average gradient a little too great for good reproduction of the average photographic print.

Though spectral sensitization of such a composition occasionally lowers the gradient, the degree of the lowering is slight, and the tone reproduction property is still too hard to produce an acceptable electrophotographic print from rather contrasty originals, such as a positive color transparency, by the positive to positive mode.

SUMMARY OF THE INVENTION An electrophotographic material suitable for the reproduction of continuous tone images is provided in the present invention. This material comprises an electrophotographic layer on a support, the electrophotographic layer comprising 'a finely-divided inorganic photoconductor, one or more spectral sensitizing dyes and an insulating film forming material. The insulating film forming material comprises an alkyd resin cured with a polyisocyanate compound. The alkyd resin must contain to 50% by weight of an aromatic dib'asic carboxylic acid, and must have a hydroxyl value within the range 10 to 100. The polyisocyanate compound is contained in an amount within the range 1.2 to 8 times the equivalent amount which corresponds to the hydroxyl value of the alkyd resin.

Accordingly, one object of the present invention is to provide an electrophotographic material suitable for reproduction of continuous tone images, and, in particular, to provide an electrophotographic material suitable for a liquid developing process with an excellent charge retention property in many insulating organic liquids.

A further object of the present invention is to disclose a novel binder composition for a finely-divided inorganic photoconductor which is spectrally sensitized.

The present inventors have solved all the above-cited various problems by using polyisocyanate compounds for the curing of alkyd resins.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows three curves which illustrates the refiectance value versus the wave length for various electrophotographic materials.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention relates to a method of forming an electrophotographic material and the material per se comprising a photoconductive insulating coating provided on a suitable support, said coating comprising a finely-divided inorganic photoconductor material, spectral sensitizer thereof, and an electrically insulating film-forming material, which is characterized 'by that said insulating filmforming material comprises alkyd resin hardened (cured) by polyisocyanate compound, said alkyd resin containing 10 to 50% by weight of phthalic or isophthalie acid, having a hydroxyl value between 10 to 100, said polyisocyanate compound being added in an amount 1.2 to 8 times as much as the amount equivalent to said hydroxyl value, and the excessive amount of said isocyanate radical being reacted with moisture.

The above electrophotographic sensitive material of the present invention gives a tone reproduction characteristic curve with a low gamma and a high sensitivity. Moreover, the change of these characteristics during storage is quite small.

Preferred alkyd resins used in the compositions of the invention 'are those which contain as the acid component in their polyester main chain aromatic dibasic acids such as phthalic or isophthalic acid, and as the alcohol component glycerin, pentaerythritol, sorbitol, ethylene glycol, etc. Such polyester chains may be modified by monobasic aliphatic acids, saturated or unsaturated, which may, then, be further modified by styrene, or acryl esters.

What is essential in the above compositions is that the polyester main chains do not substantially comprise aliphatic dibasic acids. Typical examples of suitable alkyd resins include phthalic acid-glycerin polyesters modified by any one member selected from the group comprising drying oil fatty 'acids, non-drying oil fatty acids, drying oil fatty acids modified with styrene or with acrylic ester, phenolformaldehyde resin, rosin, etc.

As will be described in the following, experimental results have disclosed that alkyd or polyester resins containing as the acid component in their main polyester chain aromatic dibasic acids give an electrophotographic coating having excellent properties. According to the descriptions in The Journal of the Japanese Technical Association of the Pulp and Paper Industry, Vol. 16, pp.

974-977, polyisocyanate is used as a hardener for a polyester resin comprising drying oil fatty acid and polyhydric alcohol, whereby the speed of photore-sponse of the resulting electrophotographic coating is lowered as the amount of the polyisocyanate increases, and greatly affected by the acid value of the polyester resin. On the contrary, if polyisocyanate is used for a polyester containing the dibasic aromatic carboxylic acid, the characteristics are not substantially varied even if the large excess of polyisocyanate is used and also the acid value of polyester does not greatly affect the characteristics of the product.

Various alkyd resins can give electrophotographic coatings of similar light sensitivity provided that they contain aromatic dibasic acids such as phthalic or i-sophthalic acid in the range between 10 to 50% by Weight of the alkyd resin (as calculated by the content of the corresponding acid anhydrides) and that they have an acid value not higher than 25. Resins which have particularly high acid values are not desirable because of slow reaction speed with isocyanates, although they are practicable if the reaction is accelerated at a high temperature. The hydroxyl value should be not higher than 100, desirably below 75, and must be above 10 for sufficient curing. Polyesters with high hydroxyl values which have widely been used together with polyisocyanates to form polyurethane coatings or the like are not suitable for the present purpose since they would produce coatings with an intense tendency to curl, mainly because of their too high cross-linking densities, and their low content of aromatic dibasic acids. When the resulting material is to be subjected to dry development processes such as cascade, magnetic brush, or powder cloud development, satisfactory dark decay characteristics can be obtained with the use of polyisocyanate 1.2 to 1.5 times equivalent to the hydroxyl content.

Suitable polyisocyanates should have a low evaporation rate at ordinary temperatures. A condensation product of 1 mol of trimethylol propane and 3 mol of tolylene diisocyanate (commercially available product: Desmodur L manufactured by Bayer Co., Ltd., Colonate L manufactured by Nippon Polyurethane Industrial Co., Ltd.) is typical.

Hexarnethylene diisocyanate, 4,4,4"triphenylmethane triisocyanate (Desmodur R of Bayer Co., Ltd), diphenylmethane-4,4'-diisocyanate (Desmodur M of Bayer Co., Ltd.), etc. can also be utilized.

The amount of these polyisocyanate compounds may be added 1.2 to 8 times the equivalent amount corresponding to the content of hydroxyl groups in the alkyd resin to be cured.

The variation of electrophotographic characteristics with the added amount of isocyanates proved very small.

On the other hand, a large amount of isocyanate is necessary to provide an electrophotographic coating which exhibits a sufiiciently slow dark decay characteristic in an insulating non-polar liquid generally used as the carrier liquid of a liquid developer. As an example in the case of a styrene-modified alkyd resin having a hydroxyl value of 50, more than twice the equivalent amount of a polyisocyanate is necessary to obtain a photoconductive coating which shows a dark decay rate in purified kerosene which is as slow as that in air. With a lesser amount than this, the dark decay rate is satisfactorily slow in air, but quite rapid in liquids such as kerosene. However, if there is employed a liquid developer with a high concentration of toner which may develop an electrostatic latent image before the charge leakage thereof, or one having a carrier liquid which has an exceptionally low dissolving power such as isoparaffinic hydrocarbon solvents, coatings containing 1.2 to 2 times equivalents of isocyanate are also practicable.

The charge decay characteristic of an electrophotographic coating in a liquid developer depends not only on the composition thereof, but also on the insulating properties and dissolving power of the liquid developer. Generally speaking, a carrier liquid having a strong dissolving power accelerates charge decay. In our experiments purified kerosene is usually used to estimate the charge decay performance of various coatings and it has been proven that the behavior in kerosene corresponds well to those in actual liquid developers, though they sometimes contain stronger solvents such as cyclohexane and have a lower electrical resistance. In the liquid developing process the slow charge decay in the developer is essential for the satisfactory reproduction of continuous tone image.

As is already known in the art, toner deposition proceeds preferentially from a high electrostatic field region independently of the use of a development electrode. Thus, in the case of development of a large, uniformly charged solid area surrounded by a background of zero charge density, development proceeds at the edge of said area, and as toner deposition proceeds, the fill-in of the area into the inner region continues, since the deposited toner at the edge of the region neutralizes the charge thereon, thus pushing the high field region inside the solid area. If the charge of the latent image leaks away during such a filling-in process, the resulting image will show a sharp density gradient decreasing from the edge towards the inner portion of the area, instead of forming a uniform density image.

Further, in the case of repulsion development wherein a liquid developer containing toner with a charge of the same polarity as that of the latent image to be developed is used, a bias voltage is applied to a development electrode facing the latent image with a close spacing, in such a manner that the field near the area having the maximum charge density in said latent image is substantially neutralized, thus preventing toner deposition at that area. When the charge decay rate of the image in the liquid developer is rapid, it is necessary to adjust the value of the bias voltage in a synchronized manner to correspondingly decrease with the decrease in the charge density in the image. This makes the apparatus complicated and causes various troubles during practice.

With a material having a slow dark decay rate in the liquid developer such control of the bias voltage is unnecessary, and, if necessary, the range of the voltage value is quite small.

Other features of the electrophotographic sensitive material of the present invention is that the tone reproduction characteristic is very soft. This has been confirmed by an experiment using a liquid developer containing a toner having an average particle diameter near 0.2 to 0.311..

When exposed to light, after being uniformly charged through an optical wedge, the electrophotographic sensitive material of the present invention yields a gamma of about 0.7 to 1.3 for a characteristic curve while the corresponding value for many conventional materials lies between 1.5 to 2.0 through similar processing. The maximum optical density of the material prepared in accordance with the present invention is generally high, reaching 1.5 to 2.0.

'y is determined from an average gradient of a tone reproduction curve which is drawn by plotting the refiection optical density of a developed wedge on the ordinate and log amount of exposure on the abscissa, whereby the length of the abscissa corresponding to the variation of times exposure and that of the ordinate corresponding to the optical density are taken to be equal, and defined as unit length. A set of two parallel lines separated by one-tenth of the abovedefined unit length is drawn in such a manner that the tone reproduction curve is tangentially put between these lines, and the slope of the lines is defined as gamma.

US. patent specification No. 3,025,160 describes an electrophotographic material utilizing a polyisocyanate such as Desmodur in the binder composition, and Harasaki and Hasegawa, et al. utilized polyisocyanate to cure unsaturated polyester resins (Journal of Paper and Pulp Technical Association 16, pp. 974-977 (1962).

The technique disclosed in the above-cited specification completely differs from that of the present invention, since it makes use of an organic solvent soluble polyurethane resin as the binder, obtained by reacting a polyisocyanate with a variety of compounds containing OH groups capable of condensing with the isocyanate radical.

In fact, it is clear that the electrophotographic materials obtainable in accordance with the cited patent are inferior to those of the present invention in respect to charge decay characteristics in insulating liquids, mainly because of their not having a three-dimensional network in their molecular structures.

On the other hand, the latter Japanese literature, which employed unsaturated fatty acids as a reaction partner, failed in providing dye sensitized materials with satisfactory properties.

Accordingly, the technique in the present invention is quite different from the known technical arts, and gives electrophotographic photoconductive coatings having excellent properties with the combination of alkyd resins and polyisocyanates.

Further, another preferable feature is that the present resin composition has no harmful influence on sensitizing dyes, thus showing no change of sensitivity under a long period of storage. It imparts a higher efiiciency of spectral sensitization to the spectral sensitizer than other resin formulations. It is difficult to explain this high efficiency at present when theories have been given only for the interaction of carboxyl and hydroxyl groups in the resin with zinc oxide, since substantially the total amount of the hydroxyl groups originally present in the alkyd resin enter into condensation reaction with the isocyanate group to give a very low concentration of hydroxyl group in the final binder structure.

Measurement of the infrared absorption of the cured coating containing zinc oxide has shown that the hydroxyl groups in the alkyd resin rap-idly react upon heating to 40 to 50 C., and then the excessive isocyanate gradually reacts with the moisture from the surrounding atmosphere, leaving a very low concentration of isocyanate.

The fact that the binder of the present invention gives high sensitivity may be understood with the explanation that although the OH groups of the alkyd resin are decreased by the reaction with polyisocyanate, the NH- groups formed by the reaction of isocyanate with water may have a similar effect on ZnO as the OH group.

Heretofore, the use of the binder containing the NH group has been relatively rare and is considered to have been overlooked because such binders are poor in other characteristics. In any event, the above explanation is only a theory.

In the case of using an alkyd resin hardened with cobalt naphthenate, spectral sensitizers seem to have been already partially broken by oxidation during film formatron, accordingly, for the case of an alkyd resin cured by an oxidizing catalyst such as cobalt naphthenate, it is clear that it is inferior to the resin formulation of the present invention in its sensitivity. Compared with other resin formulations comprising vinyl chloride/acetate copolymers, the present formulation exhibits several times hlgher sensitivity, which does not allow any clear explanation.

In a zinc oxide layer which is not sensitized with spectral sensitizers, little or no increase of photosensitivity is recognized with the binder formulation of the present invention. It should thus be noted that the feature of the present invention is demonstrated only in a spectral region expanded by the spectral sensitizer.

Other advantages of the present invention are adaptability for industrial production (because curing is carried out at a low temperature), and the capability of providing a flexible film with little tendency to curl.

Spectral sensitizers to be utilized in the present invention include all of those hitherto well known in the art, typical of which are cyanine, merocyanine, oxonol, hemioxonol, benzilidene, xanthene, sulfophthalein, triphenyl methane coloring matters, etc. Particularly, cyanin, merocyanine and Xanthenes exhibit improved characteristics with the present formulation, since they are especially sensible to oxidation.

The ratio of inorganic photoconductive material and binder is generally in the range of above 50 parts and below 50 parts of the binder per 100 parts of the former material. However, according to a special film forming method, a high insulating film is obtained even with l to 0.5 part of the binder per 100 parts of the photoconductor.

The present invention will be illustrated with the following examples in which part means by weight.

EXAMPLE 1 Herein, the process of the present invention will be compared with one of the conventional processes.

(1) The conventional process (1)a. 0.2 part of cobalt naphthenate (cobalt metal content was mixed with 100 parts of photoconductive zinc oxide and 40 parts of styrene alkyd resin varnish (non-volatile concentration 50%, phthalic anhydride content 21%, styrene content 30%, hydroxyl value 45, acid value 5, xylol solvent) in toluene, and 0.02 part of Eryshrosine (trademark, manufactured by Ciba Co., Ltd., Swiss) dissolved in a small amount of methanol was added dropwise thereto. The resulting pink colored dispersion was coated on sheets of aluminized paper; one sheet was kept in a light tight thermostat held at 40 C. for 16 hours, while another sheet was allowed to stand in a dark place at room temperature. Immediately after solvent evaporation, both sheets were measured for spectral reflectance by means of an automatic recording spectrophotometer. The heated sheet showed 62.5% reflectance at 544 mu on the basis of the magnesium oxide powder layer, while the latter sample showed only 57%.

(1)b. A sensitive layer was prepared in the same manner as in (1)-a except that part of p-dimethylaminobenzylidene rhodanine replaced the Eryshrosine B and methyl Cellosolve was used as the solvent. After coating, this was heated at 40 C. for 16 hours to cure the resin. The spectral reflectance curve is shown as curve a in the attached figure, which lacks in a clear absorption peak. Curve b shows the spectral reflectance of the same layer allowed to stand in a dark place at room temperature for four months. This shows that the rhodanine dye is completely decomposed.

Relatively low reflectance values in the total visible region is attributed to a light coating weight and the effect of the aluminum layer of the support contributing to absorption.

The photosensitivity to blue light decreased to 40% of the original value during four months storage. The blue light was given by using a blue filter passing light in the range of 380 to 510 me (no spectral sensitizer).

(l)-c. For the purpose of confirming the fact that a hardening catalyst such as cobalt naphthenate causes the above sensitivity reduction with lapse of time, the following experiment was carried out.

An electrophotographic material was prepared in a similar manner as (l)-a differing only in that cobalt naphthenate was omitted from the coating formulation. This material exhibited a reflectance value of 57% at 544 m after 16 hours storage at 40 0., showing that the sensitizer hardly decomposed in the absence of cobalt naphthenate. To confirm the harmful effect of cobalt salt there was carried out a more drastic thermal treatment of four hours storage at 60 C. with these two samples with the result that the coating containing cobalt naphthenate gave 66% reflectance, while the other showed no rise of reflectance.

The dark decay characteristics in kerosene of these drastically thermal-treated samples were measured, after complete dark-adaptation, by charging them electrostatically with a negative corona, and by wetting their surfaces with purified kerosene having a volume resistance higher than 8 10 ohms-cm. The potential remaining factor of the one containing cobalt naphthenate was 87% while that of one free of it was only 40%. After one minute dark decay, the one not containing the hardening catalyst is not completely cured even by this thermal treatment, and the charge thereon leaks away rapidly in the liquid. The temperature and moisture at the time of measurement was 22 C. and 60% RH.

(2) The process of the present invention 100 parts of photoconductive zinc oxide immersed in a methyl Cellosolve solution containing parts of p dimethylaminobenzylidene rhodanine and dried were mixed with 24 parts of the same styrene alkyd resin used in (1) diluted with a suitable amount of toluene, and, immediately before coating, 10.7 parts of a solution of polyisocyanate compound in 75% ethyl acetate (Desmodur L-75 of Bayer Co., West Germany) were added thereto, coated on an aluminized paper as in (1), and allowed to stand in a thermostat at 40 C. for 16 hours. The spectral reflectance of the layer thus obtained was as shown in curve 0 in the attached drawing. It is clear that even in (1)-b (immediately after preparation), the coloring matter has been considerably broken. In 0, the film is slightly thick, and the reflection factor is high, eliminating the influence of the dark aluminum support.

The photosensitivity for the same blue light as before was 520 for 100 in the state of a., about a five times elevation of sensitivity being obtained for (l)-b. The spectral reflection characteristic and photosensitivity were not changed after storage in a dark place for four months. Also, the potential decay in kerosene was (remaining) after one minute. In this example, 3.8 times the isocyanate of the theoretical equivalent (calculated from hydroxyl value) in alkyd resin is utilized.

EXAMPLE 2 The effect of the amount of polyisocyanate to be added to the binder composition on the electrophotographic characteristics will be described in this example.

By combining a dye sensitized zinc oxide, soybean oil modified short oil type alkyd resin varnish (non-volatile content 50%, xylol solvent, oil length 35%, phthalic anhydride content 46%, hydroxyl value 40, acid value 5), and Desmodur L-75, the following three mixtures shown in Table I were prepared.

Toluene was used as a solvent.

The adsorption sensitization for the zinc oxide surface was carried out by dispersing parts of dried zinc oxide in methanol and adding to the dispersion a methanol solution containing three different sensitizers having the following formulae:

SENSITIZER 1 S CN C=CHOH=C/ CH CHzC OOH 9 SENSITIZER 2 hr 1'31 NaO 0 Br 0 Br COONa SENSITIZER 3 S O 3 NB TABLE 1 Number A B O Z110 100 100 100 Short type alkyd varnis 32 28 Desmodur L 6. 4 8 13. 3

Each mixture was coated on an aluminum surface of polyethylene terephthalate film (vacuum coated with aluminum to a dried thickness of 7 and dried in a thermostat at 40 C. for 16 hours.

The resulting sample was dark-adapted, and, after being charged negatively, exposed to light from standard light source A through an optical wedge, whereby four pieces of the same optical wedge were parallel placed, three of which were superimposed with blue, green and red filters, respectively, in order to obtain spectral tone reproduction characteristics. After exposure, the sheet was wetted with kerosene and developed for two minutes with a liquid developer comprising a commercially available offset printing ink in cyclohexane. Development was carried out with the use of a development electrode.

Coulomb-attraction development occurred wherein the toner deposits on the area where the surface charge still remains.

The characteristic curve of the resulting image was analyzed, and the relative sensitivity and average 7 were obtained as shown in Table 2.

1 Percentage of potential remaining after dark decay for one minute under the atmosphere of 22 C. 60% EH.

2 Percentage of potential remaining after one minute dark decay in kerosene. Kerosene is applied on the photoconductive surface with a pad so as to Wet the surface.

From this table, it can be seen A is poor in decay characteristic in liquid.

10 EXAMPLE 3 The sensitive layer obtained from the coating mixture having the following composition exhibited an excellent electrophotographic characteristic:

Parts ZuO as sensitized as in Examples 1 and 2 Medium oil length linseed oil modified alkyd resin varnish (non-volatile content, 50%; oil length,

55%; phthalic anhydrous content, 33%; hydroxyl value, 20; acid number 5) Desmodur L 8 EXAMPLE 4 Substituting 28 parts of phenol resin modified short oil type alkyd resin varnish (non-volatile 50%, linseed oil length 28%, phthalic anhydride content 27%, acid number 10) for alkyd resin in Example 3, the same preferred result was obtained.

What is claimed is:

1. An electrophotographic material for use in a liquid developing environment having an electrophotographic layer comprising:

(a) a finely-divided inorganic photo-conductor,

(b) one or more spectral sensitizing dyes, and

(c) an insulating film forming material on a support,

said film forming material consisting essentially of an alkyd resin cured in the presence of moisture with a polyisocyanate compound, said alkyd resin in its cured state containing from 10 to 50%, by weight, of an aromatic dibasic carboxylic acid and having a hydroxyl value ranging from 10 to 100, and said polyisocyanate compound being present in an amount ranging from 1.2 to 8 times the equivalent amount corresponding to the hydroxyl value of said uncured alkyd resin,

said film forming material having a 3-dimensional network within its molecular structure.

2. The electrophotographic material as claimed in claim 1, wherein said alkyd resin is a styrene-modified alkyd resin.

3. The electrophotographic material as claimed in claim 1 wherein said polyisocyanate is a condensation product of about 1 mole of trimethylol propane and 3 moles of tolyene diisocyanate.

4. The electrophotographic material as claimed in claim 1 wherein said photoconductor is zinc oxide and the weight ratio of zinc oxide to binder in said electrophoto graphic material is from 3 :1 to 10:1.

5. The electrophotographic material as claimed in claim 1 wherein said aromatic dicarboxylic acid is phthalic acid or isophthalic acid.

- 6. The electrophotographic material as claimed in claim 1 wherein said alkyd resin has an acid value less than 25.

7. The material of claim 1 wherein from about 5.0 to 50 parts of insulating film-forming material are utilized per 100 parts of inorganic photoconductor.

8. The material of claim 1 wherein said alkyd resin contains as a constituent thereof a member selected from the group consisting of glycerin, pentaerythritol, sorbitol and ethylene glycol.

9. The electrophotographic material as claimed in claim 1, wherein said alkyd resin has a hydroxyl value of 75.

10. The electrophotographic material as claimed in claim 1, wherein said alkyd resin has a hydroxyl value of 100.

11. An electrophotographic material for use in a liquid developing environment having an electrophotographic layer comprising:

(a) a finely-divided inorganic photo-conductor,

(b) one or more spectral sensitizing dyes, and

(c) an insulating film forming material on a support,

said film forming material consisting essentially of an alkyd resin cured in the presence of moisture with a polyisocyanate compound, said alkyd resin in its uncured state containing 10 to 50%, by weight, of an aromatic dibasic carboxylic acid and 'having a hydroxyl value of 100, said polyisocyanate compound being present in an amount of from 1.2 to 8 times the equivalent amount corresponding to the hydroxyl value of said alkyd resin, and the acid value of said uncured alkyd resin being below 25,

said film forming material having a 3-dimensional network within its molecular structure.

References Cited UNITED STATES PATENTS 12 Sheffer et a1. 260-75 'DNK Par-k 2-60*75 TNK Bunge et a1. 260-45 TNK Greig 96-1.7 Kosche 961.5 Bunge et a1. 26075 TNK Roteman et a1 961 Stahly et al. 961.5

FOREIGN PATENTS Great Britain 961.8

CHARLES E. VAN HORN, Primary Examiner us. c1. X.R. 15 96-15; 252-501; 26022 TN, 75 NK, s73 

